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Your data is in Lat-long (EPSG:4326 by the look of it) so you are making a buffer of 1000 degrees which is going to wrap round the earth about three times!

You need to transform your spatial object to a different coordinate system based on metres (or km, or whatever units your 1000 is) using spTransform. I suspect for France there's a reasonable UTM zone with corresponding EPSG code. That will give you metres for a 1km zone with width=1000.

Note that for a whole country like France the outline may be very detailed and you might want to use gSimplify first to get a simpler outline.